1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a digital multi-functional system including a combination thereof, and more particularly, to an image forming apparatus that transfers a toner image formed on a belt member onto a transfer medium such as a recording medium.
2. Description of the Background Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charging device uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
In a color-image forming apparatus, four image carriers (which may, for example, be photoconductive drums), one for each of the colors black, yellow, magenta, and cyan, are arranged in tandem facing a belt member, that is, an intermediate transfer belt, and multiple toner images of a respective single color are formed thereon. Then, the toner images are transferred onto the intermediate transfer belt so that they are superimposed one atop the other, thereby forming a composite toner image. This process is known as a “primary transfer process”.
Then, the composite toner image on the intermediate transfer belt is secondarily transferred as a color toner image onto a recording medium in a nip, also known as a transfer nip, at which the intermediate transfer belt and a secondary transfer roller serving also as a pressing roller meet and press against each other. Subsequently, the recording medium bearing the color toner image which has been secondarily transferred thereto is conveyed to the fixing device, in which the color toner image is fixed with heat and pressure.
The intermediate transfer belt is generally wound around a plurality of rollers, one of which is a secondary transfer opposing roller disposed across from the secondary transfer roller via the intermediate transfer belt, thereby forming the transfer nip.
In a known image forming apparatus, electric discharge of the intermediate transfer belt and the secondary transfer roller occurs upstream from the transfer nip (that is, upstream in the direction of conveyance of the recording medium), yielding defective images. To prevent the occurrence of such defective images, a so-called pre-nip is formed by winding the intermediate transfer belt around the secondary transfer belt upstream from the transfer nip. In such a configuration, because the intermediate transfer belt contacts or is wound around a portion of an outer circumferential surface of the secondary roller upstream from the transfer nip to the transfer nip to form the pre-nip, electric discharge by the intermediate transfer belt and the secondary transfer roller is reduced significantly, preventing production of defective images.
Although advantageous and generally effective for its intended purpose, there is a drawback to this configuration in that when the intermediate transfer belt remains wound around the secondary transfer roller for an extended period of time, the intermediate transfer belt is deformed temporarily, that is, the intermediate transfer belt is undesirably curled along the outer circumferential surface of the secondary transfer roller. In particular, if the intermediate transfer belt remains in this condition in a hot and humid environment, curling becomes significant. The curled intermediate transfer belt does not move properly and causes problems such as distortion of the toner image primarily transferred onto the intermediate transfer belt in the primary transfer process.
To address this problem, when no image forming operation is performed, the secondary transfer roller can be separated from the intermediate transfer belt using a separation mechanism. This separation mechanism also separates the transfer roller from a photoconductive drum. However, the separation mechanism needs to separate repeatedly the secondary transfer roller from the intermediate transfer belt with a relatively large force, requiring considerable durability of the parts and components used in the separation mechanism and thus increasing its overall size and cost.
The difficulty described above is not limited to an image forming apparatus using the intermediate transfer belt arrangement. The same difficulty arises in image forming apparatuses using a belt member such as a photoconductive belt or the like, in which the belt member is wound around a pressing roller upstream from the nip to form the pre-nip.